As an anti-pollution measure, the fuel vapor that is generated in an automobile's fuel storage chambers, such as the fuel tank, the float chamber of the carburetor, etc., is usually guided to a carbon canister during stops and while traveling and adsorbed by activated carbon, which serves as an adsorbent. While traveling, air is brought into the canister to desorb the adsorbed fuel, and the desorbed fuel is fed into an inlet pipe of the engine through a control valve.
Generally, the lower is the temperature of the activated carbon, the greater is the fuel vapor adsorption capacity of the activated carbon. Conversely, the higher is the temperature of the activated carbon, the greater is its desorption capacity. However, because the reaction in which fuel vapor is adsorbed by activated carbon is an exothermic reaction with the result that the temperature of the activated carbon rises as fuel vapor is adsorbed, the adsorption capacity of the activated carbon is invariably decreased. In contrast, because the reaction in which the fuel vapor is desorbed from the activated carbon is an endothermic reaction with the result that the temperature of the activated carbon is lowered as fuel vapor is desorbed, the desorption capacity of the activated carbon is also decreased.
To solve the above problem, a canister is proposed in which a granular material having a specific heat greater than that of activated carbon is mixed with the activated carbon. In this canister, the increase in the temperature of the activated carbon caused by the heat generated by the fuel vapor adsorption from the activated carbon is attenuated by the presence of a material having a greater specific heat; conversely, the heat stored in the material having a greater specific heat is utilized to supply the heat that is required for desorbing the fuel vapor from the activated carbon, to prevent the decrease in the temperature of the activated carbon. Thus, the adsorption/desorption characteristics are improved.
However, materials having a high specific heat composed of metals, ceramics, etc., whose specific heat is low relative to the absorption/desorption heat, must be used in a large amount in combination with the activated carbon to obtain a satisfactory effect. Combined use of activated carbon with these materials having little adsorption capacity will not greatly improve the adsorption capacity as a whole, even if the temperature aspect is improved.